Nephrogenic Systemic Fibrosis (NSF) is a rare condition occurring in a small subset of patients with renal insufficiency following exposure to Gadolinium based contrast agents (GdBCA). This disorder is characterized by severe dermal and systemic fibrosis with the presence of large numbers of activated macrophages, fibrocytes and fibroblasts in affected tissues. Several published studies have reported the development of skin lesions in sub-total nephrectomized rodents injected with high doses of GdBCA, however, these lesions do not demonstrate many of the histopathological characteristics of human NSF lesions. To overcome this flaw, we propose to develop a more realistic and informative animal model of NSF by utilizing mice with adenine-induced renal failure that have either been injected intravenously with high dose GdBCA or that have been exposed to GdBCA via subdermally implanted osmotic pumps or by intratracheal instillation. Given the intense interest in the role of macrophages and, in particular, of the Toll-like receptor (TLR) pathway in the development of fibrosis, we will perform extensive mechanistic studies to examine the role of macrophages and TLRs in the development of GdBCA induced fibrosis in this new model. The hypotheses to be tested will be: 1) GdBCA exposure induces NSF-like fibrotic lesions, 2) activated macrophages are required to induce these lesions; and 3) the induction requires engagement of TLR4 and/or TLR7. To test these hypotheses, the following specific aims are proposed: SPECIFIC AIM 1: Induce fibrosis by GdBCA administration in C57BL/6J mice with adenine induced renal failure. SPECIFIC AIM 2: Investigate the role of macrophages and of TLR4 and TLR7 in the development of fibrosis in mice with adenine induced renal disease. The studies proposed here will provide valuable information regarding the initial molecular events that couple exogenous or environmental etiologic factors with inflammation and fibrosis focusing on the participation of the innate immunity system and TLR responses in these processes. Furthermore, these studies will provide important clues regarding the pathogenesis of idiopathic systemic fibrotic disorders such as SSc and may allow the identification of novel potential therapeutic targets for these diseases.